1. Field of the Invention
The present invention relates to a method of polishing a Ni--P plated aluminum alloy memory disk e a substrate for a high-capacity hard disk which uses an MR head and the like.
2. Description of the Related Art
Heretofore, a Ni--P plated aluminum alloy memory disk substrate has been finished using a double-sided polishing machine with a polyurethane polishing cloth and an alumina polishing compound by only one polishing. Recently, as the capacity of media becomes higher, a greater reduction of the roughness of the polished substrate has been required. Meeting this requirement has been accomplished mainly by reducing the average particle diameter of the polishing compound.
However, when the average particle diameter of the polishing compound is reduced, workability and productivity deteriorate, thus resulting in an increase in cost. In addition, an adverse effect arises of the waviness of the plated substrate being difficult to remove. Moreover, a further increasing of the capacity causes the amount of head float to be reduced. Even a polishing flaw (hundreds of .ANG. in depth), which is generated during the polishing and has been heretofore allowed, is defined as a failure. Therefore, a polished substrate, which simply has a small surface roughness, is not considered to be fit for use.
Accordingly, Japanese Patent Application Laid-open No. 63049/1997 discloses a polishing process as described above which is divided into two steps wherein a slurry which contains a polishing compound whose average particle diameter and maximum particle diameter are successively restricted is provided for a surface to be polished. According to the invention described in No. 3049/1997, the polishing compound is successively provided with a smaller size of the particle diameter in first and second polishing steps so as to polish the surface. Thus, in the first polishing step, the roughness on the surface to be polished is rapidly polished, and the waviness is removed at the same time. In the second polishing step, a flat polished surface is intended to be obtained. However, according to the invention described in No. 63049/1997, since polishing conditions, such as work pressure, remain as it is during the first and second polishing steps and abrasive grains, each having a different particle diameter, are changed in the same polishing machine from a large size to a smaller size so as to perform the polishing, there is a fear that the large diameter abrasive grain used in the first polishing step remains during the second polishing step, causing a problem such as a difficulty in removing the polishing flaws or the like.